Direct-viewing storage tube



S. HANSEN ET AL 4 DIRECT-VIEWING STORAGE TUBE July 2, 1957 Filed March 9, 1954 United States Patent DIRECTNIEWINGSTQRAGE TUBE .'Siegfrid Hansen, Los Angeles; Nobuo John 'Koda, Culver Si City, and yHenry-M. Smith and Sidney TrSmithJPacilic .Palisadezfgiy Calif., ,assignors-.to-. Hughes Aircraft Com- 4pany, a corporation nof Delaware :AppncatmnfMarch 9, 19s4,seria1N0.-414;99z

- ls claims. (ci. 315-13) 'Thisy invention relates to direct-viewing= storage tubes, and vmore particularly to v anL improvedt direct-viewing storage tube incorporatingapparatusV for repellingepositive ,ions away from fthe surface ,-areas employed '.to `store electric; charge.

The improved direct-viewing storage .tubefrhereinafter A,describedfis of the-same generaly type.-as.the storagextube :disclosed-aina copendi-ngy United-:.Statesfitpatent applica- .tion by Siegfried uHansen, vSerial No. :299,363,.'ztledV July 1,7, 1952, ,and entitled, Direct-Viewing#Storage Tu-be. In addition, the improved direct-viewing -storage*tube employs a'. storage grid assembly 'of Zthe-v type l'disclosed in a copending United States patent. applicati-onby Henry M. Smith,-Serial No. 409,240fled February-'9, v1954,

Y .andI entitled, Direct-Viewing StoragefTube.

-In the storage tubesof the aforementioned Hansen and .Smith.applications, Vwriting is accomplished.=byoharg ingxselected areas of .the storage grid assembly in ya positiveY direction lby means of a .high velocity electron beam. This storagey grid assembly then controls-'the ow ofabroad beam `of electrons, known -alsoas flood electronsfito a viewing screento produce a. visual image of the chargedareas on fthe storage grid assembly. In .the operation'gof these tubes, it was discovered .that'the .writing speedr-in the-central area ofthe storage grid U assembly was,considerably slower than the -writingspeed for the peripheral areas. `The cause tof thisfslower'writing speed hasy generally `been attributedto contamination ofthestorage surface areas bypositive ion bombardment since, i-n .the .manufacture of the tube, there is always some. residualgases left in the levacuated envelope. y In operating the tube,v the broad beam of electrons, which permeate substantially. the entirevolume of \theevacuy.ated-envelope, collides with t-he lmolecules `constituting the residual gases toproduce numerous 4positive ions. Inasmuchas. the composite potential ofthestoragegrid .assembly is much lower than .the .potential ofthe peripheral regions of the tube, thel potential of the central region of thetubeis more negative. than the potential of; the, peripheral regions. Thus, the positive .ionsqare attracted towards thermo-re negative central. regionof the tube which results in a more intensevbombardment bypositive ions of the central area, rather thairof-.the peripheral areas, of the storage grid.

The improved direct-viewing.storagetube ofthe present invention overcomes the aforementioned kdifficulties "by incorp'oratinga highly transparent io-n `repeller; grid .thatis `disposed coextensively with andepaced .from the storage grid assembly. .The function of theV ion. rep'eller grid..s to` isolate .the relatively lowfcomposite potential of .the storage gridassembly fromfhthe main body tof .the tube. FurthermoreJ theion repellertgrid maybe .operated so as .to actually repel positive-.ions away from the storage lsurfaces. and thus .keep 'thesesurfaceslirorn beoomingcontaminatedby the positive ions. Adirect-.viewing storage tube incorporating -the.,ion` relpeller. grid. of thepresent invention has several.- advan- "ice - ;tages,1over .fthe rtubes fdisclosed :in -rthe aforementioned #Hansen and; Smith: applications. By lway of'example,.`the writingt :speed fis; substantially uniform overrzthe lentire Atarea of the: -fstoragergridassembly,1 making: it possible. to remployxthe tube'at `its :maximum efliciency at all times. --Further, tthe .shielding of 'the storage 'gridzassemblyi eliminates the poteritialyariations*acrossthe main body-'Lof the tube. l.thus i enabling ithe flood electrons tolberdirected f .towards theastorageragrid assembly iin: a': much .more uni form: beam than in the priorztubes. :This1 generally .will

-l enable' the tube tto .i be. .made :in tmuchf i larger zsizes r :than

'previously practical. In-;addition,:.theuion -repellerVs grid enables the zfloodr electrons .1to1 beedirected tftowardsf'the storage grid assembly at substantially normallincidence, ,which minimizes .distortion :of afvisual image .'controlled It `istherefore,` anvobjectnof this: invention toprovide w an. improved; direct-viewing :storage tube having 'aunir. form fwriting :speed: over the.` entirecarea' of storage/surface.

Anotherobject .of thisiinvention Yis torprovide adire'ct- `viewing storage tube employing apparatus/'to yprevent `.contamination of'thefstorage surface'by positivetions.

Still. another fobjectfof. this-.invention isto 'provide a .direct-viewing storagetube :incorporating a highlytrans- "parentygrid torepel'positiveionsfrom thestora-ge surface.

:The :.novel featuresiwhich are believed t to be character- .istie Yofthe invention,fbothf asto l'its `organiZation-and ;method of operation;.together` withfurther :objects `and .advantages .'thereof, will..- betbetter understood fnom-` the .following description-'considered tinl connection-with the `accompanyingfdrawings in whichi an embodiment lofthe invention isfillustratedvby waynof example. It 'is to'be expresslyl understood, however,` that :the drawings, which are made a part of 'thiszdescriptiorr arel-forfthepurpose of illustration and description only, andare not intended as a definition ofithe limits'of. theinvention.

MFig. l illustrates.` a-sectional view` of. an embodiment lof; the direct-viewing #storage tube of `'the present vinvention together with associated` circuitry;

YFig. 2.illustrates=an enlarged-.sectional View of ajportion ofv thel storage .grid assembly :and associated-electrodes for the-tubev ofiFig. 1;

Fig. 3 shows awplan'vieweof a'portion of the'storage ',gridfassemblyin the tube of Fig. l; and

Fig-4 Vshows, a plan'view iofva portion of Vthe Astock- .ing.knitfion'repeller `grid in'the tube ofFig. l.

Referring to Fig. l, there is shown an embodiment rxof `the present invention .comprising anfevacuatedenvelope y10 yincluding a neck-portion` 12 that expands into aneenlargedjcylindrical portion V14,` the end offwhich isenclosedbya flat end perdoni-16. A lviewing lscreen .18 isfdisposed. onxthezinner Astiffaee of; the l'at end portion.:16 of. envelopeuliand'a st-orage grid assembly 20 together -with aihighlyntransparent-ion-repeller grid22 :are disposed coextensivewith,asrfwell asadjacent'and .-paralleltothe .viewingwscreen 18. Facing they storagezgrid assembly "20=and disposedi the i r1eck-portion` '12 fof envelope i' 10 is an 'electron gun y24lfforeproducing ahigh: energy Ielectron beam. E4`Ho'ri- `3 heating element 34, an intensity grid 36, an accelerating electrode 37, and focusing electrodes 38, 40, and 42. Heating element 34 is'connected across a source of potential 44 and has one side connected to cathode 32v as shown. Cathode 32 is maintained at a potential of the order of -3000 volts with respect to ground by means of a connection to the negative terminal of a battery 46, the positive terminal of which is connected to ground. Intensity grid 36 surrounds -the cathode 32 except for an aperture on the longitudinal axis of neck portion 12 and is maintained at a potential of from 50 to 100 volts negative relative to the potential of cathode 32 by means of a battery 48 connected in series with a resistor 49 between cathode 32 and grid 36. `An input terminal 50 is connected through a capacitor 51 to intensity grid 36 to provide a means for modulating the electron beam.

Electrodes 38, 40 and 42 provide an electrostatic lens for focusing the beam crossover formed near intensity grid 36 on the surface of storage grid assembly 20. These electrodes are annular in shape and have equal diameters of the order of one inch and the outer electrodes 38, 42 have appendages enclosing all but a center aperture on both open ends. Electrodes 38, 40 and 42 are disposed successively along and concentn'cally about the electron beam intermediate the intensity grid 36 and the storage grid assembly 20, electrodes 38 and 42 being maintained at a potential of the order of 200 volts positive with respect to ground and electrode 40 at an adjustable potential of the order of 2200 volts negative with respect to ground. These potentials are applied by means of connections from electrodes 38, 42 to a bus 60 which is maintained at 200 volts by a battery 62, and a connection from electrode 40 to a tap 64 of a potentiometer 66 which is, in turn, connected across battery 46. Accelerating electrode 37 is a Itubular element disposed concentrically about the electron beam and extending from the appendage of electrode 38 to intensity grid 36 to provide a drift space for the beam electrons. Electrode 37 is maintained at the same potential as focusing electrode 38 by being integrally attached thereto.

Horizontal and vertical electron beam deflecting means 26 and 28 comprise horizontal deflection plates 68, 69 land vertical deection plates 70, 71 which are maintained at a quiescent potential of `-I-200 volts positive with respect to ground by means of connections to bus v 60 through isolation resistors 72, 73 and 74, 75, respectively. Horizontal dellection plates 68, 69 are energized by a horizontal deflection voltage generator 76 through capacitors 77, 78, respectively and across isolation resistors 72 and 73. Similarly, vertical deflection plates 70, 71 are energized by a vertical dellection voltage generator 80 through capacitors 81, 82, respectively and across isolation resistors 74 and 75.

Electrodes 90, 92 are disposed concentrically about the inner periphery of envelope in succession from vertical deilec-tion plates 70, 71 to storage grid assembly 20. These electrodes are composed of a conductive coating, which may be provided, for example, by painting the envelope 10 with a colloidal solution of carbon such as aquadag Electrode 90 is maintained at a potential of the order of +200 volts with respect to ground by means of a connection to a bus 60 While electrode 92 is maintained at a potential of from 50 to 100 volts positive with respect to ground by means of a connection therefrom to a tap 94 of a potentiometer 96 which is connected across a battery 98, an intermediate terminal of which is connected to ground so that potentiometer 96 provides both positive and negative potentials.

Flood gun 30, in conjunction with electrodes 90 and 92, produces a broad beam of flood electrons that are directed uniformly over the entire area of storage grid assembly 20. Flood gun 30 is disposed adjacent vertical deection plate 70 and comprises a cathode-100 posi- `tioned within an intensity grid '102 which has a circular aperture in its center portion, and annular electrodes 104 and 106, disposed in succession in front of and concentrically about the aperture in intensity grid 102. Cathode 100 is operated at ground potential by means of a connection thereto while intensity grid 102 is maintained at a potential of the order of 100 volts negative with respect to ground. The latter is accomplished by means of a connection from intensity grid 102 to an adjustable tap 108 of a potentiometer 109 which is connected across a battery 112, an intermediate tap of which is connected to ground so that both positive and negative potentials are available from potentiometer 109. Electrode 106 is maintained at a potential of the order of +200 volts with respect to ground by means of a connection to bus 60 while electrode 104 is maintained at a potential of from 150 to 200 volts positive with respect to ground, depending upon the desiredy diameter of the beam of flood electrons. This potential is applied to electrode 104 by means of a connection thereto from a tap 110 on potentiometer 109.

Viewing screen 18 comprises a transparent conductive coating and a luminescent screen 122 disposed in the order named on the inner surface of end portion 16 of envelope 10. A voltage of the order of from 5,000 to 10,000 volts with respect to ground is impressed on the transparent conductive coating 120 by means of a connection to the positive terminal of a battery 124, the negative terminal of which is connected to ground.

The storage grid assembly 20 and ion repeller grid 22 are disposed approximately 1A inch apart adjacent to and in planes parallel with the viewing screen 18.

An enlarged cross sectional View showing the storage grid assembly 20 and the ion repeller grid 22 is shown in Fig. 2 and enlarged plan views of a portion of the storage grid assembly 20 and the ion repeller grid 22 are shown in Figs. 3 and 4, respectively. Referring to these figures, the storage grid assembly 20 comprises a metal ring Welded to one side of the periphery of an electroformed nickel screen 132, the screen 132 serving as a contrast control grid. A screen suitable for this purpose is of the order of 0.0005 to 0.001 .inch thick and has 250 meshes per inch with a light transparency of 40 percent. A coating 134 of dielectric material having a thickness of the order of 0.002 inch and possessing secondary electron emission is disposed uniformly over the same one side of the nickel screen 132 in a mannerto overhang the interstices of the screen and yet not clog them entirely. The light transparency of the nickel screen 132 with the coating 134 is from 20 to 23 percent. The coating 134 may be applied to the screen 132 by spraying the screen with a suspension of dielectric material such as, for example, talc simultaneously with the forcing of air through its interstices. Subsequent to coating screen 132 with the talc, it is placed in an oven land subjected to a temperature of 500 C. for l5 minutes. A thin layer of metal 136 is then evaporated over the screen 132 together with those portions of the surface of coating 134 which overhang the interstices thereof to provide a means for maintaining the potential of these surfaces the same as that of the screen 132.

A collector electrode 138 for the storage grid assembly 20 includes an annular electrode 140 and a thin conductive screen 142. Annular electrode 140 is disposed about the periphery of screen 132 just inside of ring 130 and has its edge nearest dielectric coating 134 bent under and in to form a narrow ledge 144. Thin conductive screen 142 which has of the order of from 100 to 200 meshes per inch is disposed over the surface of the dielectric coating 134 yand overlaps ledge 144 on the edges, thereby eiecting electrical contact to annular electrode 140. The exposed portions of the surface of coating 134 provide the storage surface on the storage grid assembly 20. Further, the meshes of the thin conarcanes ductive screen 142 are disposed at an angle of from 20 to` 40 with the meshes 'of nickel's'c'r'een 1-32 to minimize the moire effect on the flood electrons.

The ionl repeller grid 22'is disposed coextens'ively over the entire area of the storage grid assembly 20 'at a'distance therefrom suiiicient to establish a continuous equipotential eld across the tube. This distance will generally depend on the specific characteristics of the grid employed, 1/1 inch being satisfactory'fo'r 'the particular gridY used. A stocking knit'tu'ngsten screen, such as is shown in Fig. 4, having `a light transparency of the order of 90% and of the order'o'f 40 strands per inch is apreferable type of screen for use as grid 22. An additional requisite of grid 22 is that the wires out of which it is composed have suliicient tensile strength so it may be stretched suiciently taut across the face of storage grid assembly 20 to withstand the forces of attraction therebetween produced'by the different potentials. Ion repeller grid 22 is maintained taut across a metal ring 4150` which has an annular groove'15'2 so that it may be inserted in annular electrode 140 and remain in spaced relation therewith. Metal ring 150 is welded to an additional annulr electrode 154 which commences as an extension of annular electrode 140 and expands out to a diameter equal to the outer diameter of ring 130. Insulated supporting rods are attached to ring 130 and' annular electrodes 140, 154 in the manner shown by supporting rods 156 and y158 in Fig'. 2 to retain the storage grid assembly 20 andthe ion repeller grid 22' in correct relative position with respect to each other.

Referring again to Fig. 1, in the preferable mode of operating the tube of the present invention, contrast Control grid 13'2 is maintained at ar potential of the order of volts negative with respect to ground, collector electrode 138 is maintained at a potential from +175 to +200 volts positive with respect to ground, and ion repeller grid is preferably maintained at a potential that is positive with respect to the potential of collector electrode 138 such as, for example, +200 volts with respect to ground but, if desired, may be maintained at the same potential. This latter potential should be sufficiently high so that it predominates across the face of storage grid assembly V2'0 and, in addition, should be equal to or greater than the potential applied to collector electrode 138 so as to minimize the number of electrons intercepted by the electrode 138, since higher velocity electrons are less easily diverted. The aforementioned potentials are impressed on contrast control grid 132, collector electrode 138, and ion repeller grid l22 by connections therefrom to taps 160, 162 and 164, respectively, of potentiometer 96.

In the operation of the direct-viewing storage tube of the present invention, it is necessary to maintain a potential gradient from the storage surface of storage grid assembly 20 to the collector electrode 138 thereof. This is effected by means of flood electrons produced by nood gun 30. Flood electrons, emanating from cathode 100 of flood gun 30, which is maintained at ground potential, are directed uniformly through ion repeller-grid`22 over the entire storage grid assembly 20. The flood electrons incident on the storage surface liberate fewer secondary electrons than incident primary electrons to charge the storage surface in a negative direction until it has reached ground potential, the potential of flood gun cathode 100. In this manner, a potential gradient is produced from the storage surface provided by the surface of dielectric material 134 within the interstices of thin conductive screen 142 to the conductive screen 142 of collector electrode 138.

In the above phase of the operation, the contrast control grid 132 is sufficiently negative to allow the iiood electrons to penetrate only through the interstices within areas of the storage grid assembly wherein the storage surface is charged positive with respect to ground. In

6 order to accomplish this, however, it is necessary to allow al small' 'percentage-of the flood electrons to penetrate the areas of storage grid' assembly 20 wherein the storage surface is'at ground potential. This percentage, of course, is made as small as possible asit detrim'entally affects the contras'tfof the resultant visual image produced by the tube. Also, the meshes of the thin conductive screen 142 are disposedat an acute angle with the meshes `of contrast control g'rid132 to minimize the moire elfect on the oodelectrons Vas disclosed in the aforementioned Smith application.

Writing kon the storage g grid assembly constitutes the charging-'of storage surfaces within selected areas thereof rn a'positivefdirection by means of the high energy electr'or'rb'eam produced by electron gun 24. This electron beamis modulatedby means of a signal impressed'on input terminalSO. I 'Positively charged areas are produced on the storage surfacel by 'directing this modulated electron beam over lthe storage grid assembly 20 by deiiection 1means'26, 28'in a mannerconsistent with the charge pattern that it is'd'esiredvto produce. Each high energy electron incident on the storage surface releases numerous electrons'which are attracted to conductive screen 142 of-collectorelectrode 138 by the potential gradient to charge thev storage surface in a positive direction.

Subsequent action of the flood electrons then charges those portions of the storage'surface charged more positive than-the critical potential of ythe ldielectric material to 'the potential` of collector electrode 138 and maintains these portions at'this poten-tial for a period of time dependent on the potential applied to the collector electrode relative to the critical 'potential yof the dielectric material.

A visual image ofI the Vpositively charged areas on the storage surface is then produced on the viewing screen 18 by thel action' of -tlie flood electrons. A positive charge on the storage surface-eects field penetration through the storage grid assembly :20 to the viewing screen 18. This enables Hood electrons incident on the positively charged portions` o'f= t-liek storage surface to penetrate through the `interstices in these portions-to the viewing screen 18 to produce a 'visual image of the positively charged portion'of the storage surface.

This visual image may be retained so long as collector electrode 138 is maintained at a potential of the order of twicethe criticalpotential of the dielectric storage surface material 134. When it is desired to erase the image corresponding to the positive charged areas on the storage surface,iit isonly necessary to momentarily lower the potential of'thev collector electrode 138 to a potential lthat is less than the critical potential 'of the storage surface and then raise it gradually `back to its original value. In thisfeve'nt, the potential of the storage surface `is returned to the potential of the ood gun cathode by thel flood electrons, thereby creating a potentialy gradient from the storage surface to conductive screen 1422, thus v-malting'storage grid assembly 20 ready for writing by the high energy electron beam.

-In the'above described operationV of the direct-viewing storage tube, the-nood electrons permeating substantially the entire volume of evacuated envelope 10 produce numerous positive ions from the residual gases remaining in the tube. These positive ions converge towards the center regions of the tube due to the more positive potentials impressed on electrodes 90, 92 relative to the composite potential of the storage surface and collector electrode 138 of storage grid assembly 20. Thus, in the absence of the ion repeller grid 22, the rate of incidence of these positive ions on the storage surface in the center region of the storage grid assembly 20 is substantially greater than the rate of incidence in the outer regions. The effect of the positive ions is to contaminate the storage surface whereby, in addition to the other aforementioned detrimental results, the contaminated areas are charged in a positive direction at a considerably ,reduced rate. In the direct-viewing storage tube 4of the the more negative composite potential thereon from the main body of the tube.y Since ion repeller grid 22 is operated at a potential equal to or more positive than that of collector electrode 138, only the positive ions produced between the grid 22 and conductive screen 142 of collector electrode 138 will impinge uniformly over the storage surfaces of storage grid assembly 20. In this range of potentials, the iiow of Hood or writing electrons to the storage grid assembly is not aected. By preventing the contamination of the storage surfaces, the writing speed is not only made uniform over the entire storage` grid vassembly but made considerably faster. Furthermore, the shielding of the storage grid assembly 20 by the ion repeller grid 22 eliminates the potential variations within the main body of the tube thus enabling the flood electrons to be directed more uniformly over the storage area making the manufacture of larger tubes more feasible. Also, it is possible for the ion repeller grid to function as an auxiliary collector grid during the erasing operation. This has the effect of increasing the rate at which erasing can be accomplished and also makes spot erasing possible by eliminating the formation of a cloud of secondary electrons near bombarded areas of the storage grid assembly.

What is claimed as new is:

1. A direct-viewing electronic storage tube comprising a storage grid assembly including a collector grid on one side thereof, means for maintaining said collector grid at a iirst predetermined potential level; means, employing said collector grid, for producing a charge pattern representative of an image to be reproduced on said one side of said storage grid assembly, a viewing screen disposed in a plane parallel to and coextensive with said storage grid assembly on the other side thereof, means including a ood gun disposed in a position facing said storage grid assembly on said one side thereof for directing a broad beam of flood electrons uniformly over said storage grid assembly to act upon said charge pattern and to `penetrate through the interstices thereof to said viewing screen to reproduce said image, a transparent grid interposed between said ood gun and the collector grid of said storage grid assembly on said one side thereof, and means for maintaining said transparent grid at a potential not less than said predetermined potential level.

2. The direct-viewing electronic storage tube as deiined in claim l wherein the light transparency of said transparent grid is not less than 90%.

3. The direct-viewing electronic storage tube as defined in claim 1 wherein said transparent grid is composed of stocking knit tungsten strands.

4. A direct-viewing electronic storage tube comprising an evacuated envelope containing some residual gases;

a storage grid assembly within said envelope including a first conductive screen to provide a contrast control grid, a coating of dielectric material disposed uniformly over one side of said first screen overhanging the interstices thereof, and a second conductive screen disposed in contact with the surface of said coating of dielectric material to provide a collector grid, the portions of the vsurface of said coating of dielectric material within the interstices of said second screen constituting storage surface; means employing said collector grid for producing a charge pattern representative of an image to be reproduced on said storage surface; a viewing screen coextensive with said storage grid assembly disposed in a plane adjacent and parallel thereto on the other side of said iirst screen; means for directing a broad beam of electrons uniformly over said storage grid assembly to lact upon said charge pattern and to penetrate through the interstices thereof to said viewing screen to reproduce said image whereupon said electrons collide with the molecules constituting said residual gases to produce numerous positive ions; and means spaced from said collector grid for repelling said positive ions away from said storage surface.

5. A direct-viewing electronic storage tube comprising a storage grid assembly including a first conductive screen 'to provide a contrast control grid, a coating of dielectric material having secondary electron emission characteristics disposed uniformly over one side of said iirst screen and overhanging the interstices thereof, and a second conductive screen disposed in contact with the surface of said coating of dielectric material to provide a collector grid, the portions of the surface of said coating of dielectric material within the interstices of said second screen constituting storage surface; means for maintaining said collector grid at a predetermined potential level; means employing said collector grid for producing a charge pattern representative of an image to be reproduced on said storage surface; a viewing screen coextensive with said storage grid assembly disposed in a plane adjacent and parallel thereto on the other side of said first screen means including a ood gun disposed in a position facing said storage grid assembly on said one side of said first screen thereof for directing a broad beam of electrons uniformly over said storage grid assembly to act upon said charge pattern and to penetrate `through the interstices thereof to said viewing screen to reproduce said image; a transparent grid interposed between said iiood gun and said storage grid assembly; and means for maintaning said transparent grid at a potential no less than said predetermined potential level.

References Cited in the file of this patent UNITED STATES PATENTS 1,814,805 Hitchcock July 14, 1931 2,151,158 Schroter Mar. 2l, 1939 2,259,507 Iams Oct. 2l, 1941 2,449,569 Varian Sept. 21, 1948 2,544,755 Johnson Mar. 13, 1951 

